The present invention is directed at a process for catalytic cracking of hydrocarbon feedstocks. More specifically, the present invention is directed at a method for reducing the detrimental effects of metal contaminants such as nickel, vanadium and/or iron, which typically are present in the hydrocarbon feedstock processed and are deposited on the cracking catalyst.
In the catalytic cracking of hydrocarbon feedstocks, particularly heavy feedstocks, nickel, vanadium and/or iron present in the feedstocks become deposited on the cracking catalyst promoting excessive hydrogen and coke makes. These metal contaminants are not removed by conventional catalyst regeneration operations, which convert coke deposits on the catalyst to CO and CO.sub.2. As used hereinafter, the term "passivation" is defined as a method for decreasing the detrimental catalytic effects of metal contaminants such as nickel, vanadium and/or iron which become deposited on the cracking catalyst.
Several patents disclose the use of a reducing atmosphere to passivate cracking catalyst. U.S. Pat. Nos. 4,280,895; 4,280,896; 4,370,840; 4,372,841; 4,370,220; and 4,409,093 disclose that cracking catalyst can be passivated by passing the catalyst through a passivation zone having a reducing atmosphere maintained at an elevated temperature for a period of time ranging from 30 seconds to 30 minutes, typically from about 2 to about 5 minutes. These patents disclose that tin, antimony, bismuth and manganese may be added to improve the degree of passivation.
U.S. Pat. No. 2,575,258 discloses the addition of a reducing agent to regenerated catalyst at a plurality of locations in the transfer line between the regeneration zone and the cracking zone for countercurrent flow of the reducing gas relative to the flow of the regenerated catalyst. This patent also discloses the addition of steam to the transfer line downstream of the points at which reducing gas is added to the transfer line to assist in moving regenerated catalyst from the regeneration zone to the reaction zone. Countercurrent flow of the reducing gas relative to the catalyst flow is not desirable, particularly at relatively high catalyst circulation rates, since the catalyst and reducing gas will tend to segregate into two oppositely flowing phases. This would result in poor catalyst contacting. Moreover, it is possible that bubbles of countercurrently flowing reducing gas intermittently could interrupt the recirculation of the catalyst.
International Patent Application (PCT) No. WO No. 82/04063 discloses in the processing of metal-contaminated hydrocarbons, the addition of reducing gas to a stripping zone disposed between the regeneration zone and the reaction zone to strip the catalyst. This patent also discloses the addition of reducing gas to a separate vessel and/or to the riser downstream of the flow control means to reduce at least a portion of the oxidized nickel contaminants present.
European Patent Publication No. 52,356 also discloses that metal contaminants can be passivated utilizing a reducing atmosphere at an elevated temperature. This publication discloses the use of reducing gases for passivating regenerated catalyst before the catalyst is returned to the reaction zone. This publication also discloses that the contact time of the reducing gas with the catalyst may range between 3 seconds and 2 hours, preferably between about 5 and 30 minutes. This patent publication further discloses that the degree of passivation is improved if antimony is added to the cracking catalyst.
U.S. Pat. No. 4,377,470 discloses a process for catalytic cracking of a hydrocarbon feed having a significant vanadium content. Reducing gas may be added to the regenerator and to the transfer line between the regenerator and the reactor to maintain the vanadium in a reduced oxidation state.
European Patent Publication No. 1,642; U.S. Pat. Nos. 4,257,919; 4,326,990; and 4,324,648 also disclose the use of tin for metals passivation. These patent publications also disclose the sequential use of a reducing atmosphere at elevated temperature and the use of a hydrogen atmosphere at elevated temperature to simulate aging of the catalyst prior to testing. U.S. Pat. No. 4,235,704 also discloses the use of tin for decreasing the adverse catalytic activity of metal contaminants.
U.S. Pat. No. 4,238,317 discloses the use of a high temperature reducing zone at elevated temperature to reduce the adverse catalytic activity of metal contaminants deposited on cracking catalyst. This patent also discloses the addition of several elements or compounds including cadmium to absorb the sulfur oxides released.
U.S. Pat. Nos. 4,153,535; 4,221,677; 4,153,534; 4,206,039; 4,218,344; 4,267,072; 4,146,463; 4,233,276; 4,300,977; U.K. Pat. Nos. 1,575,018; 1,575,019; and Canadian Pat. No. 1,048,951 also disclose the use of cadmium in a catalytic cracking process to absorb the sulfur oxides released.
U.S. Pat. Nos. 4,298,459 and 4,280,898 describe processes for cracking a metals-containing feedstock where the used cracking catalyst is subjected to alternate exposures of up to 30 minutes of an oxidizing zone and a reducing zone maintained at an elevated temperature to reduce the hydrogen and coke makes. These patents describe the use of a transfer line reaction zone disposed between a regeneration zone and a stripping zone. U.S. Pat. No. 4,280,898 discloses that a metallic reactant, such as cadmium, zinc, sodium, scandium, titanium, chromium, molybdenum, manganese, cobalt, nickel, antimony, copper, the rare earth metals, and compounds of these metals may be added to adsorb the sulfur oxides produced.
U.S. Pat. No. 4,268,416 also describes a method for passivating cracking catalyst in which metal contaminated cracking catalyst is contacted with a reducing gas at elevated temperatures to passivate the catalyst.
U.S. Pat. No. 3,408,286 discloses the addition of a liquid hydrocarbon to regenerated catalyst under cracking conditions in a transfer line before the regenerated catalyst is recharged to the cracking zone. The cracking of the liquid hydrocarbon prior to entering the cracking zone operates to displace entrained regenerator gases from the regenerated catalyst entering the cracking zone.
U.S. Pat. No. 4,238,317 is directed at a method for decreasing the carbon monoxide and sulfur oxide emissions from a catalytic cracking system. A metallic oxidation promoter may be used to oxidize the carbon monoxide and sulfur oxides. The oxidation promoter may include cadmium, zinc, magnesium, strontium, barium, scandium, titanium, chromium, molybdenum, manganese, cobalt, nickel, antimony, copper, lead, the rare earth metals, and compounds thereof.
The addition of reducing gas to the transfer line between the regeneration zone and the reaction zone would obviate the necessity for installing a separate passivation vessel in the cracking system. The use of the transfer line as a passivation zone would be of particular utility in existing cracking systems where space limitations would preclude the addition of a separate passivation vessel. However, the residence time of the cracking catalyst in the transfer line is rather limited.
It would, therefore, be advantageous to have a method for increasing the rate of passivation of the metal contaminants in the transfer line.
It also would be advantageous to have a method for passivating the metal contaminants on the cracking catalyst without the addition of a separate passivation vessel.
The present invention is directed at a method for increasing the rate of metal contaminant passivation in a passivation zone disposed in a cracking system by the addition to the cracking system of a passivation promoter. The passivation promoter comprises effective amounts of elemental cadmium and/or a compound containing cadmium in combination with elemental tin and/or a compound containing tin.